Charge-coupled devices (CCDs) and CMOS (complementary metal oxide semiconductor) image sensors (CISs) are typical image sensors. A CIS is more economical than a CCD since the CIS is manufactured using usual CMOS processes. The CIS is easily integrated with analog and digital signal processing circuits.
In addition, the CIS is adaptable to operate with low-power and low-voltage such that the CIS is widely used in portable devices such as mobile phones and digital cameras. A pixel array of the CIS includes a plurality of pixels arranged in a two-dimensional matrix of rows and columns, and each pixel outputs an image signal from light energy.
A CIS with high performance including reduced noise and maintained gain in the pixels is desired.